1. Field of the Invention
Exemplary aspects of the present invention generally relate to a fixing device and an image forming apparatus including the same, and more particularly, to a fixing device using electromagnetic induction heating as a fixing method and an image forming apparatus including the same.
2. Description of the Background Art
Image forming apparatuses such as copiers, printers, facsimile machines, and multi-function machines output an image by transferring a visible image, such as a toner image borne on an image carrier, onto a recording medium, for example, a recording medium such as a sheet of paper or the like.
When the toner image passes through a fixing device, toner is fused and permeated by heat and pressure to fix the toner image onto the recording sheet.
Heating methods implemented by such a fixing device include, for example, a heat roller fixing method and a film fixing method. In the heat roller fixing method, a heat roller as a heating member having a heat generating source, for example, a halogen lamp, is disposed across from a pressure roller. The heat roller and the pressure roller are in contact with each other, thereby forming a fixing nip. In the film fixing method, a film having less heat capacity than the roller is used as a heating member.
However, as disclosed in Japanese Patent Laid-Open Application Publication 2001-13805, for example, a fixing method using an electromagnetic induction heating method has drawn attention in recent years.
In the fixing method using the electromagnetic induction heating method disclosed in Japanese Patent Laid-Open Application Publication No. 2001-13805, an induction heating coil wound around a bobbin is provided inside the heat roller. When the induction heating coil is supplied with an electric current, an eddy current is generated in the heat roller, causing the heat roller to generate heat.
Such a structure is advantageous in that a temperature of the heat roller can immediately rise to a predetermined temperature without the need to heat up as is required in the heat roller fixing method.
In the fixing method using electromagnetic induction heating, the following fixing device as disclosed in Japanese Patent Number 2975435, for example, is known. Specifically, the fixing device includes a high-frequency induction heating device and a heat-generating layer. The high-frequency induction heating device includes an induction heating coil to which a high-frequency voltage is applied by a high-frequency power source. The heat-generating layer, which is magnetic, is provided to the heat roller.
A Curie point of the heat-generating layer is set approximately to a fixing temperature. When the high-frequency induction heating device is supplied with the high-frequency voltage by the high-frequency power source, the heat-generating layer generates heat.
In such a fixing device, the high-frequency induction heating device causes the temperature of a ferromagnetic material included in an adhesive to immediately rise to the Curie point. Upon reaching the Curie point the ferromagnetic material loses its magnetism and thereafter its temperature does not rise, enabling a constant temperature to be maintained.
Since the Curie point of the ferromagnetic material is set approximately to the fixing temperature, the temperature of the ferromagnetic material is maintained at the fixing temperature. Therefore, a reduction of a start-up time of the heat roller can be attained without deterioration in separation ability (releasability) and heat resistance of the surface of the heat roller, which are required of the fixing device, and without a complicated control device. Furthermore, high-precision temperature control can be achieved.
Among heat rollers having different thicknesses and shapes of a metal core or a resin release layer thereof, the heat capacity differs as well. However, by adjusting amounts of the ferromagnetic powder material, both start-up time and temperature control precision can be improved.
Furthermore, since the ferromagnetic powder material loses its magnetism at the Curie point, toner including the magnetic powder is attracted to the heat roller, thereby preventing an offset phenomenon or the like.
In the fixing device including the magnetic heat-generating layer, the Curie point of which is set approximately to the fixing temperature, frequently a material used in a magnetic shunt layer or a layer consisting integrally of the magnetic shunt layer and the heat generating layer is highly brittle, causing cracks in the brittle layer. Such cracking is undesirable because it disrupts temperature uniformity. Consequently, there is a possibility that image formation is adversely affected.